Project Summary The prevalence of asthma and allergic diseases has been increasing for the last several decades. IgE plays a central role for the pathogenesis of asthma and allergic diseases. IL-4 and IL-13 stimulate the production of IgE. Multivalent allergen induces the cross-linking of IgE-bound high-affinity IgE receptors (FceRI) on mast cells and basophils. Cross-linking of FceRI initiates activation of several protein-tyrosine kinases (PTKs) including receptor-associated Lyn and other Src PTKs, which phosphorylate tyrosine residues of the immunoreceptor tyrosine-based activation motifs (ITAMs) in signaling subunits of the receptor. Tyrosine- phosphorylated g subunits recruit Syk, the PTK essential for triggering downstream activation events (i.e., degranulation of preformed allergenic mediators and de novo synthesis and secretion of eicosanoids and various cytokines and chemokines), eventually leading to the initiation of allergic reactions. Respiratory virus infection-induced acute exacerbations of asthma represent severe morbidity, mortality, and burdensome healthcare costs. Host cells infected with influenza A virus (IAV) sense the infection by pattern recognition receptors such as RIG-I. Signaling by RIG-I-like receptors (RLRs) occurs through the adaptor mitochondrial antiviral-signaling protein (MAVS) at the outer membrane of the mitochondria. Once activated, MAVS in turn activates TBK1 and IKKe, leading to phosphorylation and activation of the transcription factors IRF3/IRF7 and NF-kB. IRF3 and IRF7 activate the transcription of type I interferon (IFN) genes while NF-kB activates that of inflammatory cytokine genes. Mast cells are also implicated in virus infections. Mast cell production of cytokines and chemokines during IAV infection occurs in a RIG-I/MAVS-dependent mechanism, whereas histamine production occurs through a RIG-I/MAVS-independent mechanism. Mast cell- deficient mice and ketotifen (mast cell stabilizer)-treated mice showed reduced IAV-induced lung pathology compared to mast cell-sufficient mice and untreated mice, respectively. Therefore, it is likely that mast cells contribute to asthma exacerbations via the crosstalk between FceRI and antiviral signaling pathways. Our preliminary data demonstrates that MAVS, among the signaling molecules of RLR antiviral pathways, uniquely and strongly inhibits Syk activity in antigen/IgE-induced mast cell activation. Thus, Syk may be negatively regulated by a Syk- and/or MAVS-associated tyrosine phosphatase(s). In Aim 1, we will investigate whether Syk is regulated by TULA-2 or SHP-1/2 in a MAVS-dependent manner. These candidate phosphatases will be intensively analyzed for this possibility. In Aim 2, we will investigate the in vivo and in vitro roles of mast cell-expressed MAVS and Syk in IAV infection. Therefore, through this project, we will gain mechanistic insights into how Syk is regulated by MAVS in mast cells and how the crosstalk between FceRI and antiviral RLR signaling pathways impacts IAV infection. Such information will be basis for novel therapeutic/preventive interventions of IAV-induced asthma exacerbations.